1. Field of the Invention
The present invention relates to an airflow meter for detecting an airflow rate.
2. Description of the Related Art
Conventionally, an airflow meter is used for detecting rate of air drawn into an engine, for example. A detection signal produced by the airflow meter can be used for controlling various elements such as fuel injection.
For example, as shown in FIG. 4, JP 11-118557 A discloses an airflow meter 100 that includes a circuit board 101, a control integrated circuit (IC) 103 disposed on an upper surface of the circuit board 101, and a heat sink 102 attached on a lower surface of the circuit board 101. The heat sink 102 is made of a metal and protects the control IC 103 from heat generated by an operation of a transistor and the like. The heat sink 102 also functions as an attachment part of the airflow meter 100 to a passage member 105, which defines an air passage 104.
The heat sink 102 is coupled with a ground circuit part 107 through a first bonding wire 106. The ground circuit part 107 is also coupled with a connector terminal 108 through a second bonding wire 109.
In this airflow meter 100, when the heat sink 102 receives a noise from another electric equipment (i.e., external noise), the received external noise may easily enter the control IC 103. Thus, this airflow meter 100 cannot meet to a high electromagnetic compatibility (EMC) required in recent years. Furthermore, when the heat sink 102 is supplied with a high voltage by error, the second bonding wire 109 may melt down.
As shown in FIG. 5, U.S. patent application Ser. No. 11/318,553 (corresponding to JP 2006-184145 A) filed on Dec. 28, 2005 discloses another airflow meter 100, in which the heat sink 102 is not coupled with the ground circuit part 107 and floats electrically. This airflow meter 100 can prevent an external noise from entering the control IC 103. However, this airflow meter 100 cannot release static electricity stored in the heat sink 102, thereby a noise due to a charge of the heat sink 102 (i.e., internal noise) may affect the control IC 103.